patch 8.0.0206: test coverage for :retab insufficient

Problem:    Test coverage for :retab insufficient.
Solution:   Add test for :retab. (Dominique Pelle, closes #1391)
diff --git a/src/Makefile b/src/Makefile
index 1070b23..0dcf109 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2154,6 +2154,7 @@
 	test_regexp_latin \
 	test_regexp_utf8 \
 	test_reltime \
+	test_retab \
 	test_ruby \
 	test_search \
 	test_searchpos \